Method of heat treating silicon steel



Unit S Pate-fit 2,965,526 1 5 I Mnrnon OF HEAT TREATING smcon STEEL George W. Wiener, Churchill, Pa., assignorto Westing house Electric Corporation, East, Pittsburgh,-Pa., a corporation of Pennsylvania No Drawing. Filed Oct. 3, 1958, Ser. No. 765,027

4 Claims. (Cl. 148-111) lighter transformers for a given capacity, and of electrical equipment that is more economical to operate because theoperating characterisitics of the oriented ma T terials normally do not change through aging.

A significant difliculty in this art has been the inabilityj to obtain consistently satisfactorymaterial that may be,

used for these purposes. Indeed it is common practice in steel manufacturing to test a small sample of hot rolled silicon steel to determine whether it is amenable to orientation and if it is not, to dispose of the. steel as ordinary hot rolled strip. I know of no rational explanation of the inability heretofore, to orientsilicon steel of one batch and then the seemingly inconsistent, success with a succeeding batch melted to the same melting aim and prepared by the same steps as the unsatisfactory. batch.

plications. 1 n

It is a further object of the invention ,to provide a process for theproduction of orientedsilicon steel having the preferred orientation (llO)[001], that can be' practiced with existing facilities; that does not require special training for its practice; by which the percentage of grains or crystals that are preferentially oriented per unit of steel is maximized; and that consistently results in a product of magnetic properties satisfactory for high grade electrical applications.

We are taught by metal science that the magnetic or mechanical properties of any metal or alloy are determined by composition and structure. It is further known that-cornposition often influences structure so thatthey. cannot be said to be mutually independent. Therefore,

any process to improve an existing material must be" designed in such a Way that control of both composition and structure is obtained. I 7

Considering the foregoing principles in the produc tion of silicon steels having the preferred orientation ata very rapid rate.

fin Ce 2,965,526

discoveries, outstanding silicon steels characterized by the preferredorientation (110) [001] are produced by cold rolling a hotr'olled silicon steel strip having a specified composition to bring about a reduction in thickness, whereupon the resultingjstrip is annealed at an elevated temperature afterhaving been heated to that temperature The annealed strip is again cold rolled to final gauge and again annealed, advantageously at a higher temperature than the first mentioned anneal while once again heating to the annealing temperature at. a specified rapid rate. The strip may then be decarburized and given a high temperature anneal, heating to temperature at a relatively slow rate. The order of cold rolling, annealing, again cold rolling, followed by the second anneal must be carried out in. the order just specified. However, the decarburizing step and the high temperature anneal, where used, may be alternated with respect to one another as commercial conditions dictate.

In this manner I have been able to produce silicon steels having the percentage of crystals with preferred orientation (110) [001] maximized, and to grow the thus preferentially oriented crystals essentially to the exclusion, or at least comparative exclusion, of growth of other crystals that are present. Of equal importance, these advantageous results are entirely reproducible and are not the hit or-miss affair heretofore common in the art. The

materials thus produced, as will be obvious, can be utilized" l It is, therefore, an object of the present invention to provide a silicon steel strip which may be used to produce,

oriented silicon steel materials for use in electrical ap change size materially. Thus, in accordance with my in the same manner that oriented silicon steels have been used heretofore, albeit with the increased efficiencies and other advantages that attend the particular characteristics .of this material that have not been known generally here-' is essential that certain procedural limitations with re-.

spect to each of these steps be rigorously controlled if the advantages of this invention are to be obtained. The

first cold rolling operation may be'conducted in any manner desired so long as the reduction taken is within the range of about 60 'to 80 percent, and preferably to percent. be conducted with any apparatus and at any temperatureutilized heretofore. this cold roll must be in the range of about 50 to 60 percent of the then thickness of the strip. Though cold rolling, by definition, may be conducted at any temperature below the recrystallization temperature, for convenience I have found room temperature cold rolling to be particularly suitable.

The first anneal is a recrystallization anneal and, of course, it is essential that it be conducted above the re,- crystallization temperature for the material being processed. Suitably a temperature above about 850 C. is used forsteels useful in this invention. The upper temperature limit is establishedby the necessity of keeping the size of crystals small at this time; an upper limit of about-l050- C. is effective for that purpose. The anneal, which is conducted in an inert or reducingatmosphere, is brought about by heating to the annealing temperature. at a rate of about 1600 C. to 2000 C. per minute, e.g. about 1800 C. per minute. readily obtained by strand annealing or induction heating.

Orientation" is a temperature-dependent variable and, as the preferred orientation is brought about abovet ge kl 'a' tented oeezo, 1960 The second cold rolling likewise may,

However, the reduction taken in.

Such a heating rate is enemas,

recrystallization temperature while other and undesirable characteristics result upon anaealingit'lawer tempera ture, it is critically essential in the production of preferentially oriented material in accordance with my invention to heat to the annealing temperature so rapidly that appreciable softening does not occur while heating to temperature. On the other} hand, crystal growth is a fillnc iqmoi b t mperat r and time, at tsmn ratu ct, idq nhe advantagespfi mv nt' cn du, i iave a l itnecessary to avoid undue, crystal, growth, d'uri'ng'tthisf intermediate'or recrystallization anneal;"coinseqile'ntly', the; an e l n temp ra u mustbe d? in. a time; sufii c' c toeffect recrystallization but which: does, not eii I ea. one; rninute,- for it has-been found tha't the desired recrystallization will occur withinthat. time, limit and: at the tem: perature specified without incurring detrimental growth.

After the recrystallization annealing has been accomplished; thestrip'is again; cold rolleda reduction, of 50. to 60 percent-is imparted, after which the:strip, iszag ain; annealed; in an inert or; reducing atmosphere. The ternperature conditions; of; the secondv anneal, are similar to those of the; first or: recrystallization? anneal above; dc

scribed, that, is, a temperature of atleast; 850 C, to as,v V

highas about 1050 G, is -used and is obtained by utiliz ing, a heating rate-,of ;about-;160,0 C. to; 20,00; C. per minute. To avoid undesirable growth of crystals. yet. maximizethe growth of those crystals; having the preferrediorientation, it" is critically essential that the; annealing' temperaturebeattained rapidly to; get, to; the tent; peraure ,at which the preferred crystals: growi'and; to be; at-wthe inter-mediatetemperatures, where-theothen crystals; grow; a-; minimum time; It;is,for;thislreasomthat the; above-mentioned heating; rate.- is; used: in,this-;-.s econd anneal; The: material is zheldg at. the resulting: annealing temperature, for .a time: sufiicientqtog completell rsQfienz-the; material being. processed; but; not; in: excess; ofg about; one; minute;

In each of the anneals described-aboveathe silicon steeh wasbrought to the annealin g temperature at a-;. very rapid; rate. Moreover, the annealing temperature was held-for; the short period of timenotexeeeding aboutoner minute. While there are otherfeatures' of the invention, as. is apparent from this: disclosure, it, is. my: belief that the: described heating rate and hlding:;time-' are responsible: for the superior characteristicstof' the materials;produced in=this invention, .7

After the-second anneal-theastripi-may be decarburized;;- for-example, by heatingitin an=atmosphere;of'wetehyv drogen at-a-temperature of about 600C.'; 80096; and? thenl given a final annealat a=temperature up=to about 1 000" C; to 1300 =C. while -heating to'thattemperature ate-relatively slow-rate,- for example; on the-order of 2? 3. to-15? C., eL-g. 5"CI, per minute; If desiremthis'final anneal'may precede-the decarburizing step; ln'this event" the'strip is first cooled to below th recrystallization tem perature and then is heated "to the annealing temperature ju'stdescribed and isdecarburized after' annealing The entire time oftreatrnent which occurs above the recr ystallizationtemperaturein-this final anneal should be at least sufiicient for growthof thepr'eferenti-allyoriented crystals to' beadequate; i.e. for-at least'one hour yet'sh'o'uld not-be greaten than about eight hours;- A suitableprocedure that may beuSed-aften'thesecond" cold roll involves passing the; strip} through a strand ann'ealing furnace andcoili'n'g it as;ieemer eewhereupm the'resultanticoil iedee-arburized and then box-annealed applied 'are -in -theformot hot rolled p -having am ness onttheo rdei-of='0.075 to" 0. l20inch preferably hot rolled' and' coil'ed at atemperatureof not below' 1-about' 1 have -,a grain: size=ofz [mine to 3 mm": iTo in'sur'eshelf a- 7 grain size; ,I have foundpit advantageous gto; give: the, hot

rolledtpmductk s epa red nzannealtjaeaitezunerature at,

about 1050 C. to 1150 C., prior to any cold working, fbi"tftiirisiifilfiiiifttis'eellrdhafglfiifi size'i The composition of the silicon steels that are useful contain, by weight, about 2.2 to 5.25 percent silicon, 5 0.001 to 0.05 percent of material selected from the group consisting of metal oxides, metal sulfides, and metal nitrides, a maximum of abo it 0.02 percent carbon, and the balance iron and incidental materials that do not deleteriously affect the desired results. 10 While I do not desire to have my invention limited by theory, it is my belief thatl to?v secure growth of preferentially oriented, crystals while. minimizing. thegrowth of"tlie""othe'r'crystals present, itis necessary that a metalsulfide; nitride',,.or;"oxides in the defined amounts be; pres? cut in the 'steel beingatreated Indeed, the prescnce of that material in the specified amount, yetnot exceeding a 0.05 percent by weight, is critically essential to get reproducible resultsl Such impurity materials, I believe, must be those which can either bedcomposed, be re duced, or go into solid solution at the annealing temperature. Typical of such materials are copper sulfide, manganese sulfide, alumina, aluminum nitride, and silicon dioxide, Steels ofi tliiscomposition can be made according" to' a'ny processfnow used in producing electrical? steels, though} it is' preferred that after they' are hot rolled, to, the desired gauge, they are coiled at a temperature notb low about 800 C;

The"inyent1o n1 willl bedescribed. furthenin" conjunction; witli'thefollo'w gs p'ecificexam'ple. It shouldtbe ui1de'r' et'ails; disclosed'are not to beconstrued? gth'em ention.

mn dsmeen steer-albumen thick liavingacomv position,Within'therspecifiedlrange isheatedtolloof Cf. V

and, maintained; at} that": temperature: until the, average;

36 grain. size isfwithin? Lmm..to, 3; nim, The strip isftheni;

cold; rolled; at. room, temperature. using; a. conventional rp11ing;mi11;,rb reduce theIstr-ipto a thickness orio oso,

inch. The reducedstrip ispassedthrough a strand an nealing furnace,- having' a dry hydrogenatmosphere, maintainediat..a'temperature of9501 C. at arate suflicient to, insure that the vstrip remains in the furnace for a period of: one minute; The strip is then again cold rolled to a, thickness of 0.014 inch, at room temperature whereupom the resultant strip is; again strand annealed in dryhydro,

ger at9 C, for one minute or the timerequired just to: softengthematerial Asthe strand,leaves the annealing v furnace it is coiled and'is placed in a furnace through:

which-wet hydrogen is circulated continuously. The,

temperature tin, the furnace is maintained ,below 800 C 50 After one-half, hour; in; the d ecarburizing' furnace, the. coiled strip is again annealed in a box annealing furnacei heating to 12 00 C. at a temperature increase of 5.3 Gperl minute. The atmosphere during thislatter: anneal .is maintained inert and the anneal iscontinued for:

fourrhoursa, Subsequent'tests of this material as determined by X-ray analysis, torque-curves,'magnetic characte'r, and crystallographic studies show the steel to. have theprefe redor enta n F a dth t the crvs tals so oriented extend essentiallylthroughout the material; 00 In other tests silicon steels produced in accordancewith this invention ,were obtained and tested; A sample-having V V afinal thicknessof 0.012 inch andran analysis as follows;-

' V Y 7 Percent" Sf? L H J v v 292% Q v 0 0053:

Fe B'aIanc'e' 7 hja'd the ,following,magnetie properties:v

Goer'eiveiorce; 5 a 011-25 7 Maximum nermeab'ilitvi .52',0O(Y Residual induction -from a:tip induction of -1 5,000,: r

' gaus's 1 12,200.

Hysteresis :losszimavat n n htp rpreteena e 009% A sample of the same material was also tested for total watt loss per pound at 15,000 gauss at various frequencies. The data are:

W-60 .59 W-120 1.66 W-180 3.00 W-240 4.84 W-320 6.86 W-360 8.91

From these data, it will be readily apparent that materials produced in accordance with my process set forth above are unusually useful for various electrical applications.

In accordance with the provisions of the patent statutes, I have explained the principles of my invention and have described what I now believe to be its best embodiment. However, it should be understood that variations from the details disclosed may be made without departure from the spirit and scope of my invention.

I claim as my invention:

1. In a method of producing oriented silicon steel, the step which comprises cold rolling a hot-rolled silicon steel strip of a thickness of about 0.075 to 0.120 inch having a composition consisting essentially, by weight, of about 2.2 to 5.25 percent of silicon, 0.001 to 0.05 percent of at least one material of the group consisting of metal oxides, metal sulfides and metal nitrides, a maximum of about 0.02 percent of carbon, and the remainder iron, to reduce said strip about 60 to 80 percent, heating said cold rolled strip at a rate of about 1600 C. to 2000 C. per minute to a temperature of about 850 C. to 1050 C. and maintaining said strip at said temperature for a time sufiicient to effect recrystallization, cold rolling the thus heattreated strip to elfect a further reduction of about 50 to 60 percent, based on the then thickness, and again heating said strip at a rate of about 1600 C. to 2000 C. per minute to a temperature of at least about 850 C. and maintaining said temperature for a period of time just sufficient to soften the material, then subiect ng the strip to a high temperature anneal for a period of from 1 to 8 hours at a temperature of from 1000 C. to 1300 C., said high temperature being attained by heating at a rate of from 2 C. to C. per minute, each of said anneals being conducted in a non-oxidizing atmosphere and subjecting the strip to a decarburizing anneal at a temperature not exceeding 800 C. in wet hydrogen, the order of precedence of the high temperature final anneal and the decarburization anneal being immaterial.

2. A method according to claim 1 in which said recrystallization anneal is effected by strand annealing said strip for a period not in excess of one minute.

3. A method according to claim 1 in which said hot rolled silicon steel strip is annealed at a temperature of about 1050 C. to 1150 C. for a period of time sufficient to have the average grain size within about 1 mm. to 3 mm. prior to said first cold working operation.

4. A method of producing oriented silicon steel which comprises cold rolling a hot-rolled silicon steel strip of a thickness of about 0.075 to 0.120 inch having a composition consisting essentially, by weight, of about 2.2 to 5.25 percent of silicon, 0.001 to 0.05 percent at. least one material of the group consisting of metal oxides, metal sulfides, and metal nitrides, a maximum of about 0.02 percent of carbon, and the remainder iron, to reduce said strip about to percent, heating said cold rolled strip at a rate of about l600 to 2000 C. per minute to a temperature of about 850 to 1050 C. and maintaining said strip at said temperature for a time suflicient to efiect recrystallization but not in excess of about one minute, cold rolling the thus recrystallized strip to efiect a further reduction of about 50 to 60 percent, based on the then thickness, again heating said strip at a rate of about 1600 C. to 2000 C. per minute to a temperature of at least about 850 C. and maintaining said temperature for a period of time to just soften the sheets, then decarburizing said strip at a temperature below about 800 C., and high-temperature annealing it at a temperature of about 1000 C. to 1300" C. said high temperature annealing temperature being attained by heating to temperature at a rate of 2 C. to 15 C. per minute, each of said anneals being conducted in a non-oxidizing atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 2,112,084 Frey et al. Mar. 22, 1938 2,113,537 Hiemenz Apr. 5, 1938 2,287,467 Carpenter June 23, 1942 2,307,391 Cole et a1. Jan. 5, 1943 OTHER REFERENCES Iron Age. volume 170, Nov. 20, 1952, pages -142, 144, by Turner et al, 

1. IN A METHOD OF PRODUCING ORIENTED SILICON STEEL, THE STEP WHICH COMPRISES COLD ROLLING A HOT-ROLLED SILICON STEEL STRIP OF A THICKNESS OF ABOUT 0.075 TO 0.120 INCH HAVING A COMPOSITION CONSISTING ESSENTIALLY, BY WEIGHT, OF ABOUT 2.2 TO 5.25 PERCENT OF SILICON, 0.001 TO 0.05 PERCENT OF AT LEAST ONE MATERIAL OF THE GROUP CONSISTING OF METAL OXIDES, METAL SULFIDES AND METAL NITRIDES, A MAXIMUM OF ABOUT 0.02 PERCENT OF CARBONS, THE REMAINDER IRON, TO REDUCE SAID STRIP ABOUT 60 TO 80 PERCENT, HEATING SAID COLD ROLLED TRIP AT A RATE OF ABOUT 1600* C. TO 2000* C. PER MINUTE TO A TEMPERATURE OF ABOUT 850* C. TO 1050* C. AND MAINTAINING SAID STRIP AT SAID TEMPERATURE FOR A TIME SUFFICIENT TO EFFECT RECRYSTALLIZATION, COLD ROLLING THE THUS HEATTREATED STRIP TO EFFECT A FURTHER REDUCTION OF ABOUT 50 TO 60 PERCENT, BASED ON THE THEN THICKNESS, AND AGAIN HEATING SAID STRIP AT A RATE OF ABOUT 1600* C. TO 2000* C. PER MINUTE TO A TEMPERATURE OF AT LEAST ABOUT 850* C. AND MAINTAINING SAID TEMPERATURE FOR A PERIOD OF TIME JUST SUFFICIENT TO SOFTEN THE MATERIAL, THEN SUBJECTING THE STRIP TO A HIGH TEMPERATURE ANNEAL FOR A PERIOD FO FROM 1 TO 8 HOURS AT A TEMPERATURE OF FROM 1000* C. TO 1300* C., SAID HIGH TEMPERATURE BEING ATTAINED BY HEATING AT A RATE OF FROM 2*C. TO 15*C. PER MINUTE, EACH OF SAID ANNEALS BEING CODUCTED IN A NON-OXIDIZING ATMOSPHERE AND SUBJECTING THE STRIP TO A DECARBURIZING ANNEAL AT A TEMPERATURE NOT EXCEEDING 800* C. IN WET HYDROGEN, THE ORDER OF PRECEDENCE OF THE HIGH TEMPERATURE FINAL ANNEAL AND THE DECARBURIZATION ANNEAL IMMATERIAL. 